1. Field of the Invention
The present invention relates to a magnetic lens having a long focal length for the optical imaging of a specimen having a large surface area, i.e., a specimen having a diameter of several centimeters, by means of a charged-particle beam in an image plane which comprises a cylindrical coil including one winding, or alternatively, a plurality of separately-excitable windings, which is surrounded by a field-carrying metallic shell member fabricated of iron disposed at the radially outer surface of the winding.
2. Description of the Prior Art
Magnetic lenses of the foregoing type comprising a cylindrical coil including a single winding are known in the art and may be utilized, for example, as a field lens in an electron beam-optical reducing apparatus. See the journal "Optik", Volume 28, No. 5 (1968/1969), pages 518 through 531. Integrated circuits are fabricated using magnetic lenses of this type by imaging a transmission mask on a wafer.
A magnetic lens including a cylindrical coil comprising a plurality of separately-excitable windings is also known in the art. See U.S. Patent Application Ser. No. 805,474 filed June 10, 1977 for "Improved Long-Focus Magnetic Lens". This lens enables (a) the focal length to be changed while the excitation is maintained constant and (b) the entire field pattern to be shifted in the axial direction while the lens is fixed in space.